The present invention relates to a method for making tetraorganooxysilanes. More particularly, the present invention relates to a process involving the reaction of a natural silicon dioxide source in the presence of an organo carbonate.
Tetraorganooxysilanes are silicon-containing compounds of the formula (RO).sub.4 Si where R is an alkyl group, aryl group or mixture thereof. Tetraorganooxysilanes include tetraalkoxysilanes, tetraaryloxysilanes, and mixed tetra(alkoxyaryloxy)silanes. Silicon-containing compounds, such as tetraorganooxysilanes, are commonly made using manufactured silicon dioxide as a starting material. Unfortunately, manufactured silicon dioxide is not an energy efficient source of silicon. Hence, different sources of silicon to synthesize silicon-containing compounds are constantly being examined.
The process commonly used commercially for the production of silicones and more particularly, alkoxysilanes, was first described by Rochow et al., U.S. Pat. No. 2,473,260. The Rochow process uses silicon, also referred to as elemental silicon, as a starting material. The elemental silicon must first be reduced from silicon dioxide. The elemental silicon is then oxidized to yield alkoxysilanes via a reaction of the silicon with methanol in the presence of a copper catalyst. It is well known in the art that the silicon-oxygen bond in silicon dioxide is extremely stable. In order to break the silicon-oxygen bond, a large amount of energy is consumed when silicon dioxide is reduced to elemental silicon. Thus, due to the large amount of energy needed to break the silicon-oxygen bond, the synthesis of silicones from silicon dioxide and the Rochow process is expensive and not energy efficient.
In other work related to the invention, several complex compounds have been studied for the synthesis of silicon-containing compounds. Rosenheim et al. (Z. Anorg. Allg. Chem. 1931, 196, 160) described the formation of hexacoordinated dianionic complexes from silica under basic conditions. Silica, sand and quartz powder were depolymerized in the presence of alkali catecholates.
Other methods for the synthesis of silicon-containing compounds have been described which do not use silicon dioxide as a starting material. Laine et al. (Nature 1991, 353, 642) published a method for synthesizing pentatcoordinate silicates from silica, ethylene glycol, and base. The pentacoordinate silicate produced is a highly reactive compound which can be a useful precursor of new silicone compounds.
Ono, Akiyama and Suzuki (Chem. Mater. 1993, 5, 442) reported that silica gel reacts with gaseous dimethyl carbonate (DMC) at 500.degree. K. to 600.degree. K. to yield tetramethoxysilane in the presence of a catalyst supported on the silica. Ono et al. (Inorg. Chim. Acta 1993, 207, 259) also determined that rice hull ash, which has 92% silicon dioxide purity, also reacts with dimethyl carbonate in the presence of a catalyst at 625.degree. K. However, silica gel as well as rice hull ash are manufactured materials and do not provide significant cost advantage over the well-established route to tetraalkoxysilanes through elemental silicon.
In the past, the synthesis of silicon-containing compounds has relied heavily on the reduction of silicon dioxide to elemental silicon. Unfortunately, the large amount of energy needed for synthesizing silicones such as tetraorganooxysilanes from silicon dioxide can be problematic. Thus, new synthetic routes are constantly being sought which rely on an efficient energy source of silicon dioxide.